It is well known that solar energy may be used to provide hot water, and this is common in tropical and sub-tropical countries. In higher latitudes the gain is less but still beneficial. In Britain, for instance, solar energy can provide domestic hot water for an average family through the summer months and a useful top-up (by pre-heating the water supply) through the rest of the year. Over the course of a year a south-facing roof in Britain receives about 1000 kWh/m2, which is clearly an important potential source of energy.
Typically the solar energy is captured by means of a so-called flat plate collector which is often mounted upon a roof. The collector comprises a closed box with a transparent cover and, inside the box, an absorber plate coloured black or otherwise treated to absorb incident radiation. Water passed through pipes within the box is heated. The amount of energy captured depends upon the size of the collector, which is naturally limited by handling and aesthetic considerations, particularly when the collector is roof-mounted. Also, importantly, the effectiveness of the collector depends upon limiting heat loss, and as well as being thermally insulated the box is conventionally closed to prevent convective heat loss. Thus, in use, still air provides a blanketing effect so that heat builds up in the box.
It is previously known to cover a roof with photovoltaic tiles to generate electricity, but it will be appreciated that these are relatively easy to connect together. For a thermal system, however, conventional flat plate collectors, if reduced to the size of a typical roof tile, would entail unacceptable time and cost in interconnection. Accordingly various previous proposals have been made to adapt roofing tiles to deliver solar heating.
Offenlegungsschrift DE 3934719 discloses an arrangement in which aluminium tiles are shaped to hook over pipes of a solar heating system, which pipes thereby serve as tiling battens. The tiles are anodised and have a matt black outer face so as to absorb incident solar radiation and pass heat on to water circulating through the pipes. This system lacks any insulative covering for the tiles, which would thereby suffer considerable heat loss from convection, and even more so when subject to precipitation and afterwards due to evaporation.
Offenlegungsschrift DE 3218013 has a tiling batten which is itself is made of thermally conductive metal and embraces a pipe for a solar heating system. However it is proposed that conventional roofing tiles be hung from these battens, which restricts the solar gain because conventional tiles are not transparent to solar radiation. A further reduction of effectiveness stems from the fact that the battens cover only a comparatively small part of the roof area which could be used for collecting solar heat.
U.S. Pat. No. 4,202,319 discloses roofing tiles in the form of reduced-size flat plate collectors. Each of these tiles has an inlet and an outlet connected to channels in a batten, thus providing a circulatory route for fluid of a solar heating system. Those skilled in the science will appreciate that the need to ensure a fluid-fight connection between each tile and its supporting batten places great demands on both the accuracy of manufacture and the skill of the roofer. This implies high cost, which is raised further by coils or a similar heat collecting structure within each tile.
A much simpler approach is proposed in Japanese Patent No. 4 343 963 whereby heat exchange pipes are laid within grooves formed in fibreboard roofing sheets which are then covered with transparent tiles. However with the arrangement disclosed there is no provision for the desirable blanket of still air.
U.S. Pat. No. 4,364,374 discloses a tile-like panel with a plurality of spaces for air. However this air circulates (for the purpose of heat transfer) rather than acting as a blanket. Further, the panel is designed and arranged in such a way that connection to a solar heating system is provided by means of a pipe extending through one of the spaces which, when the panel is connected to others on a pitched roof, makes an open passage for air from the base to the summit of the roof. This necessarily entails substantial convective heat loss.
U.S. Pat. No. 4,083,360 discloses an arrangement in which one or more flat plate collectors are disposed between rafters of the roof, beneath an area of radiation-transmitting tiles. The tiles may be formed, in the manner of double glazing, to provide heat insulation, and it is stated that the collectors may extend throughout the slope of a roof.
United Kingdom Patent 2 070 232 discloses a roof structure for collecting thermal energy in which air passes through ventilation elements under tiles of the roof. The tiles are conventional, so they are not transmissive of solar radiation as in a normal collector, and the system provides for an air flow rather than a blanket of substantially still air.